# Birmingham gauge

The Birmingham gauge, officially the Birmingham Wire Gauge and often abbreviated as G or ga, is unit or wire gauge used to measure the thickness or diameter of wires and tubing, including hypodermic needles and other medical tube products.

## Terminology

The Birmingham gauge is also known as the Stubs Iron Wire Gauge or Birmingham Wire Gauge and is distinct from the Stubs Steel Wire Gauge and the British Standard Wire Gauge. It is commonly referred to simply as gauge (abbreviated as G), but this should not be confused with the French gauge, a separate system used for measuring the outer diameter of catheters.

## System

The Birmingham gauge ranges from 5/0 or 00000, the lowest gauge number corresponding to the largest size of 0.500 inches (12.7 mm), to 36, the highest gauge number corresponding to the smallest size of 0.004 inches (0.10 mm). The increments between gauge sizes are not linear and vary. At higher gauge numbers, the increment between the two highest gauges is 0.001 inches (0.025 mm), while at lower gauge numbers, the increment between the two lowest gauges is 0.046 inches (1.2 mm). This progression does not follow a mathematical formula but instead reflects historical manufacturing practices in the wire drawing industry, where the size increments were determined by practical tooling, such as a draw plate, and production constraints. In applications involving wires and fine tubing, the gauge number refers to the product's outside diameter. For larger mechanical tubing, however, the gauge number indicates the wall thickness, independent of the tube's overall size.
In medicine, the Birmingham gauge is widely used to specify the outer diameter of hypodermic needles, catheters, cannulae, and suture wires. However, catheters are more commonly defined using the French catheter gauge. The Birmingham gauge was originally developed in early 19th-century England for wire manufacturing and became common in medical settings in the early 20th century.
The Birmingham gauge system is also recognized in international medical standards, such as ISO 6009:2016, which includes a color-coding system for hypodermic needles. This standardization helps ensure accurate identification and compatibility of medical equipment.

## Sizes of hypodermic needles

Hypodermic needles are available in a wide variety of outer diameters described by gauge numbers. Smaller gauge numbers indicate larger outer diameters. Inner diameter depends on both gauge and wall thickness. The following chart shows nominal inner diameter and wall thickness for regular-wall needles. Thin-wall needles (not shown) have identical outer diameters but larger inner diameters for a given gauge.
| Gauge, G | Nominal outer diameter | Nominal outer diameter | Nominal inner diameter | Nominal inner diameter | Nominal wall thickness | Nominal wall thickness | ISO 6009 color |
| Gauge, G | (inch) | (mm) | (inch) | (mm) | (inch) | (mm) | ISO 6009 color |
| 7 | 0.180 ± 0.001 | 4.572 ± 0.025 | 0.150 ± 0.003 | 3.810 ± 0.076 | 0.015 ± 0.001 | 0.381 ± 0.025 | (undefined) |
| 8 | 0.165 ± 0.001 | 4.191 ± 0.025 | 0.135 ± 0.003 | 3.429 ± 0.076 | 0.015 ± 0.001 | 0.381 ± 0.025 | (undefined) |
| 9 | 0.148 ± 0.001 | 3.759 ± 0.025 | 0.118 ± 0.003 | 2.997 ± 0.076 | 0.015 ± 0.001 | 0.381 ± 0.025 | (undefined) |
| 10 | 0.134 ± 0.001 | 3.404 ± 0.025 | 0.106 ± 0.003 | 2.692 ± 0.076 | 0.014 ± 0.001 | 0.356 ± 0.025 | Olive brown |
| 11 | 0.120 ± 0.001 | 3.048 ± 0.025 | 0.094 ± 0.003 | 2.388 ± 0.076 | 0.013 ± 0.001 | 0.330 ± 0.025 | Green-yellow |
| 12 | 0.109 ± 0.001 | 2.769 ± 0.025 | 0.085 ± 0.003 | 2.159 ± 0.076 | 0.012 ± 0.001 | 0.305 ± 0.025 | Pale blue |
| 13 | 0.095 ± 0.001 | 2.413 ± 0.025 | 0.071 ± 0.003 | 1.803 ± 0.076 | 0.012 ± 0.001 | 0.305 ± 0.025 | Purple |
| 14 | 0.083 ± 0.001 | 2.108 ± 0.025 | 0.063 ± 0.003 | 1.600 ± 0.076 | 0.010 ± 0.001 | 0.254 ± 0.025 | Pale green |
| 15 | 0.0720 ± 0.0005 | 1.829 ± 0.013 | 0.0540 ± 0.0015 | 1.372 ± 0.038 | 0.0090 ± 0.0005 | 0.229 ± 0.013 | Blue-grey |
| 16 | 0.0650 ± 0.0005 | 1.651 ± 0.013 | 0.0470 ± 0.0015 | 1.194 ± 0.038 | 0.0090 ± 0.0005 | 0.229 ± 0.013 | White |
| 17 | 0.0580 ± 0.0005 | 1.473 ± 0.013 | 0.0420 ± 0.0015 | 1.067 ± 0.038 | 0.0080 ± 0.0005 | 0.203 ± 0.013 | Red-violet |
| 18 | 0.0500 ± 0.0005 | 1.270 ± 0.013 | 0.0330 ± 0.0015 | 0.838 ± 0.038 | 0.0085 ± 0.0005 | 0.216 ± 0.013 | Pink |
| 19 | 0.0420 ± 0.0005 | 1.067 ± 0.013 | 0.0270 ± 0.0015 | 0.686 ± 0.038 | 0.0075 ± 0.0005 | 0.191 ± 0.013 | Cream |
| 20 | 0.03575 ± 0.00025 | 0.9081 ± 0.0064 | 0.02375 ± 0.00075 | 0.603 ± 0.019 | 0.00600 ± 0.00025 | 0.1524 ± 0.0064 | Yellow |
| 21 | 0.03225 ± 0.00025 | 0.8192 ± 0.0064 | 0.02025 ± 0.00075 | 0.514 ± 0.019 | 0.00600 ± 0.00025 | 0.1524 ± 0.0064 | Deep green |
| 22 | 0.02825 ± 0.00025 | 0.7176 ± 0.0064 | 0.01625 ± 0.00075 | 0.413 ± 0.019 | 0.00600 ± 0.00025 | 0.1524 ± 0.0064 | Black |
| 22s | 0.02825 ± 0.00025 | 0.7176 ± 0.0064 | 0.00600 ± 0.00075 | 0.152 ± 0.019 | 0.01110 ± 0.00025 | 0.2826 ± 0.0064 | (undefined) |
| 23 | 0.02525 ± 0.00025 | 0.6414 ± 0.0064 | 0.01325 ± 0.00075 | 0.337 ± 0.019 | 0.00600 ± 0.00025 | 0.1524 ± 0.0064 | Deep blue |
| 24 | 0.02225 ± 0.00025 | 0.5652 ± 0.0064 | 0.01225 ± 0.00075 | 0.311 ± 0.019 | 0.00500 ± 0.00025 | 0.1270 ± 0.0064 | Medium purple |
| 25 | 0.02025 ± 0.00025 | 0.5144 ± 0.0064 | 0.01025 ± 0.00075 | 0.260 ± 0.019 | 0.00500 ± 0.00025 | 0.1270 ± 0.0064 | Orange |
| 26 | 0.01825 ± 0.00025 | 0.4636 ± 0.0064 | 0.01025 ± 0.00075 | 0.260 ± 0.019 | 0.00400 ± 0.00025 | 0.1016 ± 0.0064 | Brown |
| 26s | 0.01865 ± 0.00025 | 0.4737 ± 0.0064 | 0.00500 ± 0.00075 | 0.127 ± 0.019 | 0.00680 ± 0.00025 | 0.1734 ± 0.0064 | (undefined) |
| 27 | 0.01625 ± 0.00025 | 0.4128 ± 0.0064 | 0.00825 ± 0.00075 | 0.210 ± 0.019 | 0.00400 ± 0.00025 | 0.1016 ± 0.0064 | Medium grey |
| 28 | 0.01425 ± 0.00025 | 0.3620 ± 0.0064 | 0.00725 ± 0.00075 | 0.184 ± 0.019 | 0.00350 ± 0.00025 | 0.0889 ± 0.0064 | Blue-green |
| 29 | 0.01325 ± 0.00025 | 0.3366 ± 0.0064 | 0.00725 ± 0.00075 | 0.184 ± 0.019 | 0.00300 ± 0.00025 | 0.0762 ± 0.0064 | Red |
| 30 | 0.01225 ± 0.00025 | 0.3112 ± 0.0064 | 0.00625 ± 0.00075 | 0.159 ± 0.019 | 0.00300 ± 0.00025 | 0.0762 ± 0.0064 | Yellow |
| 31 | 0.01025 ± 0.00025 | 0.2604 ± 0.0064 | 0.00525 ± 0.00075 | 0.133 ± 0.019 | 0.00250 ± 0.00025 | 0.0635 ± 0.0064 | White |
| 32 | 0.00925 ± 0.00025 | 0.2350 ± 0.0064 | 0.00425 ± 0.00075 | 0.108 ± 0.019 | 0.00250 ± 0.00025 | 0.0635 ± 0.0064 | Deep green |
| 33 | 0.00825 ± 0.00025 | 0.2096 ± 0.0064 | 0.00425 ± 0.00075 | 0.108 ± 0.019 | 0.00200 ± 0.00025 | 0.0508 ± 0.0064 | Black |
| 34 | 0.00725 ± 0.00025 | 0.1842 ± 0.0064 | 0.00325 ± 0.00075 | 0.0826 ± 0.019 | 0.00200 ± 0.00025 | 0.0508 ± 0.0064 | Orange |
| 1 2 Not strictly defined, and may vary for a given gauge. ||||||||
Rapid blood transfusion through 23G or smaller needles can cause hemolysis (rupturing of red blood cells).

## Sizes of catheters

This includes peripheral venous catheters. The gauge compared to outer diameter is the same as for needles, but the color coding is different.
| Gauge | Outer diameter (mm) | Maximum flow rate (ml/min) | Color |
| 14 | 2.0 | 250–300 | Orange |
| 16 | 1.7 | 180 | Grey |
| 18 | 1.3 | 75–120 | Green |
| 20 | 1.1 | 40–80 | Pink |
| 22 | 0.9 | 42-55 | Blue |
| 24 | 0.7 | 20-35 | Yellow |
| 26 | 0.6 | 10-13 | Black |